When you come out of Heuston station, stay on the same side of the river and head into the city along the quays, passing the Guinness Brewery on Victoria Quay. Continue along then to Ushers Quay until the end of Merchants Quay. At the end of it, turn right onto Winetavern Street and up the hill passing Christ Church Cathedral. Walk straight across the large junction onto Nicholas Street. Take the first turn left onto Ross Road. At the end of it, turn right onto Bride Street and the next turn left is Chancery Lane.
Alternatively, coming out from Heuston Station you could go up Steeven's Lane, where the Luas line is. At the top turn left and walk along James's Street and then Thomas Street and onto High Street, which will bring to the large junction where Christ Church is, at which point you would turn right onto Nicholas Street and proceed as above.
It is not a very long walk either way, perhaps just 15 or 20 minutes, but you could also take a taxi. See the map below, showing Heuston Station at the left end, before Victoria Quay, and Chancery Lane at the right end, marked with the A.